One of the largest groups of parasites, the trematodes, have all kinds of strategies to trasmit between hosts. The genus Microphallus causes its hosts like shrimp, to swim upwards or otherwise endanger themselves so that they are eaten by the parasites next host - birds.
Sure, it looks harmless.
But perhaps the coolest trematode is Dicrocoelium dendriticum. It has a very unusual lifecycle, which is between grazing animals like sheep, snails, and ants. The adults in the sheep's liver reproduce and shed eggs into the feces. The eggs are then picked up by snails, which apparently are fairly fond of eating sheep poo. The larvae then hatch and multiply, finally leaving the snail as little slimy balls that are irresistible to ants.
But then, you might ask, how does it get from ant back into sheep? Sheep don't eat ants, and ants don't tend to interact with sheep. So the fluke manipulates the ant's brain in a superb form of body-snatching. At night, infected ants climb to the top of grass blades and bite down, waiting. If they're eaten by a sheep, great, if not, in the morning the ant climbs down and resumes its normal activities, only to return to the grass blades at night. Because the ant (and thus the parasite) wouldn't survive all day in the sun, the parasite selectively controls the ants behavior only at night.
If you aren't totally amazed by the precision of this behavior, you should be. Imagine if we had such precise control of someone's neurons and behavior! It's a sci-fi dream come true. Luckily, it doesn't infect our brains...